A CRT, such as a color picture tube or a color display monitor, includes a phosphor screen which is composed of different color-emitting phosphor elements, each of which emits a different color of light when impacted by electrons. The phosphor elements are formed on an interior surface of a faceplate panel by one of the known screen processing techniques. In the formation of the phosphor elements, the color selection electrode assembly is used as a photographic master. The color selection electrode has a plurality of small apertures therethrough to facilitate screening printing, and also to permit the passage of electron beams during the operation of the CRT. The color selection electrode assembly is detachably mounted within the faceplate panel, in proximity to the phosphor screen, by affixing the frame thereof to mounting means secured to the faceplate panel. During the screen processing operation, the color selection electrode assembly is inserted into and removed from the faceplate panel a number of times, thereby increasing the probability that phosphor, glass, or metal particles may become attached to the surface of the color selection electrode or block some of the apertures. After the screening operation, a thin metal film, such as aluminum, is deposited over the phosphor elements to reflect light emitted therefrom outwardly through the faceplate. The metal film also provides an electrode to which an electron accelerating potential can be applied. The faceplate panel is then frit sealed to a funnel portion of the CRT envelope, at an elevated temperature, in a suitable sealing furnace. When the sealed panel-funnel assembly leaves the sealing furnace, it cools and pulls in air. This greatly enhances the possibility that airborne contaminants, such as glass particles, metal flakes, dust particles, small fibers, etc., can be introduced into the envelope and block the apertures of the color selection electrode, or become lodged in gaps between the color selection electrode and the frame.
In the operation of the CRT, an electron beam is provided for each of three primary color-emitting phosphors. The three electron beams converge at the color selection electrode and pass through the apertures thereof and impact a phosphor of the proper light-emitting color. If an aperture of the color selection electrode is blocked by a conductive particle, an objectionable spot is apparent on the screen. On the other hand, insulative particles, which are charged negatively by the electron beams, will cause deflection of the beams by coulomb repulsion. Therefore, the insulative particles can cause picture imperfections, such as screen spots, when attached to the color selection electrode without physically blocking the apertures. Furthermore, it has been observed that the insulative particles, in addition to causing screen spots, also cause color misregister of the electron beams. The color misregister creates a "halo" effect resulting from the electron beams being deflected by the negatively charged particles and striking the phosphor elements adjacent to the obscured region. Additionally, the conductive particles can fall into the electron gun of the completed tube and cause electrical shorts or arcing.
It is known in the art to attempt to dislodge the objectionable particles by thumping the faceplate panel or ultrasonically vibrating the envelope while purging the envelope with a charged fluid, such as ionized air; however, such expedients have not substantially reduced the number of objectionable particles, as found by analyzing tubes rejected for particle-related problems.